Bahamut was an ancient esper of enormous power, looking mostly like a huge metallic gray dragon.

He formed The Empire to lay waste to those who would oppose him, starting with Truce. He negotiated a contract with Deneb for her support and to conscript a gaggle of War Witches into his already-formidable army.

After a particularly grueling battle, Deneb attempted to negotiate a cease-fire. Enraged at her apparent cowardice, Bahamut attacked her, breaking the contract and transforming him into a chunk of Magicite, which was her plan all along. After this, The Empire quickly dissolved and Deneb signed a peace treaty with Truce.

What's left of him is probably on a necklace hanging between Deneb's boobs, which is not such a terrible place to spend the rest of eternity I guess.